If you want to pay to be the only person in the Facebook VR world to own a Yacht, you need a legal contract that gives you ownership of that. If you don't have a contract you cannot sue him for selling Yachts to other people inside of his VR world.
Now if you built a VR application where objects were only displayed if you could prove ownership of the private keys that would work, but there would be nothing stopping someone from modifying the VR application, or building their own VR application to give you access to a Yacht.
